"I think the problem here for Letterman is the story is not over," Washington Post media reporter Howard Kurtz told "Good Morning America" today. ""I think this is going to continue to dribble out because there is a criminal case at the heart of it."

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Despite a boost in ratings since the scandal broke out, Kurtz cautioned that viewership could trail off after the initial surge of interest dwindles.

"We're talking about sex with subordinates," he said, adding that Letterman can't afford to be seen as a "creepy old man." "Over time, this could chip away at Letterman's image, and he knows it."

Joe Halderman, 51, an Emmy-award winning producer for CBS News' "48 Hours," has pleaded not guilty to charges he tried to blackmail Letterman for $2 million. Halderman allegedly claimed he had evidence of the talk show host's sexual affairs with staff members.

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Halderman's attorney, Gerald Sharge,l told "Good Morning America" Monday that he looked forward to cross-examine Letterman on the stand and that the alleged extortion plot was "preposterous."

But noted defense attorney Roy Black told "Good Morning America" today that the defense has a difficult task, as offering proof of Letterman's alleged sexual affairs may only strengthen the prosecution's case.

"The worse the secret is, the more it improves the offense," he said. "That's not a defense. It just helps prove the crime."

Apologizes to Wife

Letterman changed his mind about keeping Monday's show free of chatter about the scandal and used part of his monologue to apologize to his wife, Regina Lasko, the mother of his 6-year-old son.

"She has been horribly hurt by my behavior, and when something happens like that, if you hurt a person and it's your responsibility, you try to fix it," he said. "And at that point, there's only two things that can happen: either you're going to make some progress and get it fixed, or you're going to fall short and perhaps not get it fixed."

According to a source close to Letterman, the relationships Halderman was allegedly going to reveal occurred before Letterman's March 2009 marriage to Lasko.

Letterman also apologized to his staffers who have subsequently been put in an awkward spot.

"I'm terribly sorry that I put the staff in that position," he told the studio audience. "The staff here has been wonderfully supportive to me, not just through this furor, but through all the years that we've been on television and especially all the years here at CBS, so, again, my thanks to the staff for, once again, putting up with something stupid I've gotten myself involved in."

Letterman, Steve Martin Poke Fun at Sex Scandal

The audience seemed largely sympathetic Monday, laughing and applauding as he made jokes about how he spent his weekend, "raking his hate mail."

"I would give anything to be hiking on the Appalachian Trail," he joked, in a reference to South Carolina Gov. Mark Sanford, whose dramatic extramarital affair had been a previous prime target for Letterman.

"I got in the car this morning and the navigation lady wasn't speaking to me," he cracked. Keeping up the patter, Letterman added, "There's a possibility that I will be the first talk show host impeached," an reference to former president Bill Clinton's fling with a White House intern.

Guest Steve Martin also took a jab at the comedian.

"I was going to come out and make a few jokes, but it's really not funny -- a little bit, a little bit funny," he said, "but I do think that the one thing that will come out of this mess that has come out already, it proves that you are a human being and, you know, we weren't really that sure before."

Letterman told his audience that he "has his work cut out for him" in dealing with the fallout from his on-air confession.

'Not So Fast'

Shargel, Halderman's lawyer, told "Good Morning America" Monday that Letterman's on-air confession last week was hardly the final word on the matter.

"Joe Halderman was at CBS for 27 years. Here's a guy who was an investigative journalist for so many years," Shargel said. "To say he was trapped in an extortion plot was kind of preposterous."

"I'm here to say, not so fast," Shargel said. "I look forward to cross-examining David Letterman, because I don't think the full story is before the public. There's much more to this story."

Freed on $200,000 bail, Halderman faces up to 15 years in state prison if convicted.

Who is Joe Halderman

Last week, Letterman revealed to the studio audience that he'd received a package three weeks ago containing a threat to reveal those indiscretions if he didn't pay up.

In a press conference last week before Halderman's arraignment, Manhattan District Attorney Robert Morgenthau said Halderman demanded to be paid $2 million in an initial meeting with Letterman Sept. 9, when Halderman allegedly waited outside Letterman's New York City apartment.

Halderman had threatened to reveal "personal and private information" if Letterman did not accede to his demand to purchase a one-page screenplay he presented at the initial meeting, said Morgenthau.

The district attorney said the screenplay described a scenario in which Letterman's world would "collapse all around him" and would lead to a "ruined reputation."

Three subsequent meetings between Letterman and Halderman were recorded by Letterman's attorney, Morgenthau said.

Shargel told "Good Morning America" that he had not yet had a chance to hear the tapes that prosecutors say prove his client tried to extort Letterman, nor read the transcripts.

And while Shargel admitted that Halderman did attempt to cash Letterman's phony $2 million check, "the surrounding circumstances are what's relevant," Shargel said.